Sen. Marco Rubio of Florida said Thursday that 2016 GOP rival Donald Trump is a “touchy,” “insecure” guy unfamiliar with key issues and prone to simply lashing out when things aren’t going his way.

“Well, first of all, he takes shots at everybody who gets anywhere close to him in terms of a poll, or anytime he hits a rough spot. That’s what he does,” Mr. Rubio said during an appearance on Kentucky Sports Radio.

“He had a really bad debate performance last week, he’s not well-informed on the issues — he really never talks about issues and can’t have more than a 10-second sound bite on any key issue,” Mr. Rubio said. “And so I think he’s kind of been exposed a little bit over the last seven days, and he’s a very touchy and insecure guy. So that’s how he reacts, and people can see through it.”

The remarks came hours after Mr. Trump, speaking on CNN, labeledMr. Rubio as an overly ambitious “kid” who shouldn’t be running for president, after going after the senator on Wednesday in South Carolina as well.

Mr. Rubio had been asked why voters should choose him in a hypothetical head-to-head match-up with Mr. Trump. He also said he was confident his message of modernizing policies and turning the page from “outdated leaders” would resonate with Americans across the political spectrum.

“My argument is if we can modernize our policies, turn the page from outdated leaders and really embrace these new policies for the 21st century, America’s going to be better than it’s ever been,” he said. “And if we don’t, we’re going to be the first Americans to leave our kids worse off than” ourselves.

“And I just feel like in this race, I’m the only one that’s talking about it this way,” Mr. Rubio continued. “And most certainly [in] comparison to the individual you just mentioned, that’s true, but in general. It’s one of the reasons I got in this race. I honestly believe that that message is one that appeals to Americans across the political spectrum.”